Mr. Seidner, who was a generalist fund manager at Pimco and a member of its investment committee, will join as a partner and start March 4, GMO said Monday in an e-mailed statement. The firm manages $12 billion in fixed income, according to its website. Tom Cooper, previously head of fixed income, will continue to lead the firm's emerging-debt strategies.

GMO is known for the bearish views of its chief investment strategist Jeremy Grantham, who correctly forecast in 2000 that U.S. stocks would decline in the coming decade, and as early as July 2007 predicted that a large global bank would go bust amid credit market declines. Mr. Seidner, who worked with Mr. El-Erian at Harvard University's endowment before joining him at Pimco, left last week as the firm named six deputies to investment chief Bill Gross.

At Pimco, Mr. Seidner worked on the separately managed accounts affiliated with mr. Gross's $237 billion Total Return Fund (PTTRX), as well as low duration and alpha overlay strategies. Mr.Gross said in an interview last week that Pimco rehired Sudi Mariappa as a generalist fund manager to take over responsibilities for Mr. Seidner.

Mr. Seidner was Pimco's interim head of global equities after Neel Kashkari left in January 2013. That role was filled when Pimco in October announced it hired Virginie Maisonneuve from Schroders to oversee the unit.

Before joining Pimco in 2009, Mr. Seidner was a domestic fixed-income portfolio manager at Harvard Management Co., the manager of the university's endowment, which Mr. El-Erian headed from 2006 to 2007. Mr. El-Erian said last month he's stepping down from his roles as CEO and co-chief investment officer at Pimco.